Fire detectors



Sept. 4, 1956 A. F. KRUEGER 2,762,033

' FIRE DETECTORS Filed Sept. 23, 1955 '5 '40 ,I5 320 E j I3 I00 5 FREQ(cps) I9 I? RADIATION SWEEP DETECTOR AMPLIFIER MIXER 4 0 c ATOR f, to f f to f 260 To 300cps 11 la/ V [1 fgl +0 E +fg FILTER FIG. I TUNED 2| AT 250 cps CLIPPING FIRE CIRCUIT ALARM 23 29 INTEGRATING 0U T P UT 25/ CIRCUIT CIRCUIT FLAME SWEEP MIXER FLICKER OSCILLATOR OUTPUT FREQUENCIES FREQUENCIES FREQUENCIES CPS CPS CPS 265 250 a 280 270 250 a 290 275 250 a 300 so 280 250 a 3IO 300 250 a 350 NVENTOR ALBERT F. KRUGER F I G. 2

ATTORNEY United States Patent ,0

FIRE DETECTORS Albert F. Krueger, Needham, Mass., assignor to Electronics Corporation of America, Cambridge, Mass., a corporation of Massachusetts Application September 23, 1955, Serial No. 536,197

4 Claims. (Cl. 340-227) This invention relates to fire detectors.

In order not to be subject to false alarms, a fire detector should detect some physical phenomenon exclusively associated with fires in the environment in which the detector is to operate. Oneof the properties of flames is that the radiant energy emanating therefrom is modulated in amplitude. In other words, flames flicker, and this flicker occurs at frequencies in the sub-audio and lower-audio ranges. A satisfactory fire detector distinguishes flames from other sources of flickering radiation by means of a band-pass filter which makes the detector sensitive to flicker within lower and upper frequency limits only. The lower-frequency limit, which may be of the order of five cycles per second, prevents the actuation of the fire detector by very low-frequency transients occurring, for instance, when Venetian blinds are lowered. The upper-frequency limit, which may be of the order of twenty-five cycles per second, prevents the actuation of the fire detector by the flicker of lamps energized by alternating current. An integrating device is also provided to prevent the actuation of the fire detector until the occurrence of several cycles of flicker within the bandpass frequencies. The great advantage of this fire detector is its speed of response when compared with that of fire detectors which, instead of detecting flames, detect some secondary effect of fire, such as heat.

This fire detector is therefore actuated by several successive cycles of flicker within the band-pass frequencies of five to twenty-five cycles per second. Although these flicker frequencies occur very seldom in radiation sources other than flames, they may still be found, for instance, in the reflection of light from the blade of a slowly-rotating fan. This fire detector may therefore give an accidental false alarm. It may also purposely be made to give a false alarm by chopping a light beam directed against it with ones hand.

It is the principal object of this invention to provide a fire detector which is much less likely to be falsely alarmed than previous fire detectors of the flicker type.

Flame flicker comprises a plurality of frequencies having different amplitudes. A very rough estimate of the relationship between the amplitude and the frequency of flicker components emanating from a typical fire would be that, above one cycle per second, the amplitude of a flicker component is halved when the frequency is doubled. The amplitude of the flicker components above, let us say, fifty cycles per second is so small that these components can be disregarded. Thus flame flicker is characterized in that it comprises many different flicker frequencies occurring simultaneously.

The fire detector in accordance with the present invention operates a fire alarm only when it detects many different flicker frequencies which occur simultaneously. It comprises a radiation-sensitive device to convert the flickering radiation emanating from a flame into an electrical signal the amplitude of which varies as a function of flame intensity. It comprises also a local oscillator designed to sweep a given range of frequencies. The

2,762,033 Patented Sept. 4, 1956 output signals of the radiation detector and of the oscillator are applied to a mixer. The output of the mixer is applied to a filter which is tuned sharply. As the oscillator frequency is varied, there is a variation of the flicker frequency which, when mixed with the oscillator frequency, will give an output signal which is passed by the filter. Thus, the filter will give a continuous output signal only if the radiation detector detects many flicker frequencies occurring simultaneously. The fire alarm is made responsive to this continuous output. I

Other and incidental objects of the present invention will be apparent to those skilled in the art from a reading of this specification and an inspection of the accompanying drawings in which:

Figure 1 is a block diagram of a fire detector in accordance with the present invention; and

Figure 2 is a table of frequencies illustratingthe operation of the fire detect-or shown in Figure 1.

Referring to Figure 1, the fire detector comprises a radiation detector 11. This radiation detector is designed to convert the flickering radiation emanating from a flame into an electrical signal the amplitude of which varies as a function of the intensity of said radiation. The radiation detector 11 may comprise, for instance, a bolometer or a photoelectric cell. -It has been found that lead sulfide photocond-uctive cells are particularly suitable for the detection of flame radiation.

As we have seen, flame flicker comprises flicker frequencies in a range from, let us say, 1 to 50 cycles per second, the amplitude of the lower flicker frequencies being much greater than the amplitude of the higher ones. In the presence of a flame, therefore, the output signal of the radiation detector 11 fluctuates, this fluctuation comprising slow variations of high amplitudes and faster variations of much lower amplitudes. 1

The output signal of the radiation detector 11 is passed through an amplifier 13, the gain of which varies with frequency in accordance with the graph 15. This graph shows the relative gain increasing between 1 and about 50 cycles per second, then dropping very sharply. T he characteristics of amplifier 13 are such that its output signal, when the radiation detector 11 detects a flame, is characterized by variations in amplitude of approximately the same magnitude in a range of frequencies fr-fz from a few to about 50 cycles per second.

The fire detector also comprises a local sweep oscillator 17 which is designed to sweep a range of frequencies from a lower frequency is to a higher frequency f4. For example, the sweep oscillator 17 may sweep frequencies in a range from 260 to 300 cycles per second at a rate of 1 cycle per second. The output signals of the amplifier 13 and of the sweep oscillator 17 are fed to a mixer 19. The output signal of the mixer 19 is two beat frequencies, equal to the difference between and the sum of the frequencies applied thereto by the amplifier 1'3 and the sweep oscillator 17. The output frequencies of the mixer 19 range therefore from the lowest dilference of the frequencies applied thereto, i. e. (f3f2), to the sum of the highest frequencies applied thereto, i. e. (f2+f4).

The output signal of the mixer 19 is applied to a sharply-tuned filter 21. This filter 21 may be tuned, for instance, at 250 cycles per second, which is outside the range of frequencies swept by the oscillator 17. In other words, the filter 21 passes only signals having a frequency of 250 cycles per second. The output signal from the filter 21 is fed through a clipping circuit 23 to an integrating circuit 25. The output signal from the integrating circuit 25 is applied to an output circuit 27 which actuates a fire alarm 29.

The operation of the fire alarm'shown in Figure 1 will be explained by reference to the table shown in Figure 2.

As the sweep oscillator frequencies vary from 260 to I 300"cycl es-per-;second, "there is a single frequency detected bytheradiationdetector '11 which, when mixed-withthesweep oscillator frequency,,wil1 give a beat frequency of 2,5 0 cycles per second. In othenwords, the filter 21 will provide-"a continuous output signal orily ithere is impressed upon "radiation 'detector' l-i a.;comppsite ,"sign'al comprising all the frequencies between 10 and SOcycles per second.- On the other hand,;if th'ere'is "impressed operates the fire alarm 29;

' Thus-the'fire -'alarm-'29 is operatedonly when the radiation detector 11 detects many different-flicker frequencies,- :in thera-nge of -10 to ;50cycle s-per second, occurring simultaneously. This reduces considerably the chances of-false alarms.

Ic'laim;

1; A fire detector comprising a radiation detector to,

convert-the fli'ckering'radiation emanating from a flame into-"an electrical signal the amplitude of-which varies-as afunetion of the intensity =offlsaid radiation, an oscillator designed to sweep a range of frequencies,- a mixer, means toa'pply tosaid-mixer the output sign-alswof said oscillator and :ofsaid radiation detector, a filter tuned at -a frequency outside the range of frequencies of said oscillator,

means -to-.apply to said filtertheoutput signal :of said outputof said mixer, and a'fire'alarmresponsive to the tiller.

2; A fire detector comprising a radiation detector'to convert the flickering radiation emanating from a flame intoan electricalsignal the amplitude of 'which'varies 'as a function of the intensity of said radiation and over a,

ranger-of -frequencies ranging from a lower frequency f1 I to a higher frequency is, an oscillator designed to sweep arangeof frequenciesranging from-alower frequency-fa to a higher frequency ii, a mixer, means to apply to said mixer the output signals of said oscillator and of said radiation detector, a filter tuned at a frequency not lower than (fa-f2) and not higher than (fz+f4), means to apply the output signal of saidmixer to said filter, and a fire alarm responsive to, the outputlof said filter.

*3. A fire detector comprising a radiation detector to convert, the flickering radiation emanating'from a flame into; an electrical signal the amplitude of'which variesyas a function of the intensity of said radiation and'over a range of-frequenciesiranging from a lower frequency f1 to a higher frequency f2, an oscillator designed to sweep a range of frequenciesrangingfrom a lower frequency fa to a higher frequency ii, a mixer, means to apply to said mixer the output signals of said oscillator and of said radiation detector, a filter tuned at a frequency not lower than (fa-f2) and not higher-than (fag-f4) but outside the range between f3 and f4, meansto'apply the, outputsignal offsaid miXer to saidfi'lter, and a fire alarm responsive to the output ofsaidfllter. p

4.- A fire detector comprising aradiat-ion detectorjto convert the flickering radiation emanating from aflame into an'electrica'l signal the amplitude of "which varies as a function of the intensity of said radiation and over a.

range-of-frequencies ranging from a lowerfrequ'ency f1 .to

-a higher fre quency fz,;an' oscillator designed'tosweep arange of frequencies-ranging from a lowerfrequen'cy f3 to-abigherfrequency f4, a mixegmeansto apply-to said mixer the output signals of'said oscillator and of said radiation-detector, a filter tuned at-a-frequency notjlower than;(fsf2) and-nothigher than (f2+f4), means'to apply the output signal'of: said mixerto said filter, an

integrating circuit, meansto apply the output signal of said. filter to said integrating circuit,- .and'a fire. alarm responsive to the output of said integrating circuit.

References Cited in the fi-le of this patent UNITED STATES PATENTS, 2,722,677i

Krueger Nov. 1, 1955 

